let x = 100000
x.ToString "#,#" |> printfn "%s"
x.ToString "N0" |> printfn "%s"
printfn $"{x:N0}"

In fsi, this prints 3 identical lines based on the current culture.

In Fable, the result is

100,000
100000
%P(N0)

• x.ToString "#,#" produces the correct string, but I'd appreciate a Fable warning saying that invariant culture will be used (I think I've seen something like that in other places).
• x.ToString "N0" silently produces an incorrect string. An unsupported format error is warranted in my opinion.
• $"{x:N0}" creates the compiled representation of the interpolation string, which is complete nonsense. An unsupported format error is warranted in my opinion.
• Also $"{1000:``#,#``} items" results in %P(#,#) items.